core: Continue PieCrust3 refactor, simplify pages. The asset pipeline is still the only function pipeline at this point. * No more `QualifiedPage`, and several other pieces of code deleted. * Data providers are simpler and more focused. For instance, the page iterator doesn't try to support other types of items. * Route parameters are proper known source metadata to remove the confusion between the two. * Make the baker and pipeline more correctly manage records and record histories. * Add support for record collapsing and deleting stale outputs in the asset pipeline.

---
theme_config:
    site:
        title: "Some Test Theme"
in:
    pages/foo.md: "This is: {{site.title}}"
outfiles:
    foo.html: "This is: Some Test Theme"
---
theme_config:
    site:
        title: "Some Test Theme"
in:
    pages/foo.md: "This is: {{foo}}"
    configs/theme_preview.yml: "foo: bar"
outfiles:
    foo.html: "This is: bar"